In present power management devices, such as power management controllers and/or chargers, switches can be used to direct power, e.g., direct power from a power source to a system load or to a rechargeable battery pack. In power management controllers or chargers, switches are generally implemented by bipolar transistors or Metal-Oxide-Semiconductor Field Effect transistors (MOSFETs). In electrical circuitries, ideal switches which are defined as having zero ON-state resistances and infinite OFF-state resistances are desired. MOSFETs can have relatively lower ON-state resistances and relatively higher OFF-state resistances than other types of switches.
Generally, a P-channel MOSFET (PMOS) switch may be driven in an ON state by biasing a gate terminal voltage of the PMOS switch to a low voltage level (e.g., 0 volt) with respect to the voltage on a source terminal of the switch. To turn on an N-channel MOSFET (NMOS) switch, a driving voltage at a gate terminal of the NMOS switch may need to be substantially greater than a source voltage at the source terminal (e.g., 5 volts greater than the source voltage). In conventional circuitries, the source terminal of an NMOS switch may be coupled to a positive terminal (or an output) of a power source (e.g., a battery pack). Therefore, the driving voltage for the gate of the NMOS switch may need to be substantially greater than the output voltage of the power source. This intrinsic characteristic of NMOS switches can limit their applications since such a high driving voltage may not be available. Consequently, PMOS switches are used extensively in current power management devices.
Although easier to drive, PMOS switches may have substantially larger ON-state resistances than NMOS switches having the same sizes as PMOS switches. For example, the ON-state resistance of a PMOS switch can be two times larger than the ON-state resistance of an NMOS switch having the same size. Accordingly, power dissipation of switches can be doubled if PMOS switches are employed instead of NMOS switches.
To reduce power dissipation of PMOS switches and obtain targeted power transfer efficiencies, PMOS switches with low ON-state resistance may be employed. However, such PMOS switches are costly as they may need a special fabrication process. Furthermore, such PMOS switches may also need extra chip area to accommodate drivers to drive them. Therefore, costs of such power management devices are increased.